Various analytical instruments can be used for analyzing proteins and other biomolecules. More recently, mass spectrometry has gained prominence because of its ability to handle a wide variety of biomolecules with high sensitivity and rapid throughput. A variety of ion sources have been developed for use in mass spectrometry. Many of these ion sources comprise some type of mechanism that produces charged species through spraying. One particular type of technique that is often used is Electrospray Ionization (“ESI”). One benefit of ESI is its ability to produce charged species from a wide variety of biomolecules such as proteins. Another benefit of ESI is that it can be readily used in conjunction with a wide variety of chemical separation techniques, such as High Performance Liquid Chromatography (“HPLC”). For example, ESI is often used in conjunction with HPLC for identifying proteins.
Typically, ESI produces a spray of ions in a gaseous phase from a sample stream that is initially in a liquid phase. For a conventional ESI mass spectrometry system, a sample stream is pumped through an electrospray device, while a relatively high electric field is applied between an end of the electrospray device and an electrode that is positioned adjacent to the end of the electrospray device. As the sample stream exits through the end of the electrospray device, surface charges are produced in the sample stream, thus pulling the sample stream towards the electrode. As the sample stream enters the high electric field, a combined electro-hydrodynamic force on the sample stream is balanced by its surface tension, thus producing a “Taylor cone.” Typically, the Taylor cone has a base positioned near the end of the electrospray device and extends up to a certain distance away from the end of the electrospray device, beyond which a spray of droplets is produced. As these droplets move towards the electrode, coulombic repulsive forces and desolvation lead to the formation of a spray of ions in a gaseous phase.
Characteristics of a Taylor cone typically depend on an affinity between a sample stream and a surface at an end of an electrospray device. Depending on this affinity, a greater or lesser area of the surface can be wetted by the sample stream, which, in turn, can affect a volume of the Taylor cone. A Taylor cone with a larger volume can present a number of disadvantages. In particular, the larger volume of the Taylor cone can translate into a larger volume of a sample stream that is required for mass spectrometric analysis, which can be problematic when analyzing proteins and other biomolecules that are present in small quantities. Also, the larger volume of the Taylor cone can create a “dead volume” within which internal fluid circulation can occur. Within this “dead volume,” distinct bands of sample streams can merge, thus compromising band resolution. In addition, the larger volume of the Taylor cone can reduce ionization efficiency. Accordingly, for these and other reasons, it is desirable to reproducibly produce Taylor cones with low volumes, such that results of mass spectrometric analysis have a desired level of accuracy, reproducibility, and sensitivity.
Recently, attempts have been made to implement polymeric devices as electrospray devices for use in mass spectrometry. Such polymeric devices are desirable, since they can be precisely shaped using a wide variety of techniques. One of the challenges to successfully implementing such polymeric devices relates to an affinity between a sample stream and polymeric materials that are typically used to form these polymeric devices. In particular, this affinity can promote formation of Taylor cones with larger volumes, which can be disadvantageous for the reasons described above.